Regarding differences, it seems the Neo4j folks have spent a lot more time than we have on make it a “database” (while InfoGrid delegates to other storage engines like MySQL or Hadoop).
On the other hand, InfoGrid.fnd is type-safe, and instead of a command-line shell, uses a set of web Viewlets to access the graph of objects. (Which then can be incrementally refactored into an application.)
And then of course there is InfoGrid.net, which does not seem to have an equivalent in Neo4j. (see also InfoGrid Core features and Neo4j wiki)